1. Field of the Invention
This invention relates to a miniature fixed-focal-length lens system which is especially suitable for use in a video camera or a still-video camera in which a solid state image sensor is employed, and more particularly to such a fixed-focal-length lens system which can be used as a dual-focal-length lens system in combination with an afocal wide convertor which is selectively positioned in front (the object side) of the fixed-focal-length lens system to switch the focal length of the overall lens system.
2. Description of the Prior Art
There has been known a dual-focal-length lens system which comprises a stationary lens group and a movable lens group and whose focal length can be switched between two values by positioning the movable lens group in front of the stationary lens group and removing therefrom. Such a dual-focal-length lens system is frequently employed in a video camera or a still-video camera, a video camera which is also capable of taking a still photo. The movable lens group is typically an afocal lens system which is generally called `convertor` and generally comprises a first lens group having a negative refracting power and a second lens group having a positive refracting power. Further the lens of the stationary lens group nearest to the object is generally a positive lens convex to the object.
In a video camera or a still-video camera, a solid state image sensor (e.g., CCD) is frequently employed and when the angle of incidence of the imaging light to the image plane is large, eclipse occurs at the light incident end of each element of the image sensor and a so-called shading is generated in reproduced images. In order to prevent generation of shading, the exit pupil of the lens system must be at a distance from the image surface so that the angle of incidence of the imaging light to the image plane is small.
For such a purpose, the stop must be positioned on the object side of the lens system.
That is, referring to FIGS. 7A and 7B, when the stop P is inside the lens system L as shown in FIG. 7A, the stop P is remote from the front focal point and as a result the exit pupil is positioned close to the image surface I. On the other hand, when the stop P is positioned on the object side of the lens system L as shown in FIG. 7B, the stop P is positioned close to the front focal point and as a result the exit pupil can be positioned remote from the image surface I.
In the aforesaid known lens system, the stop is positioned inside the stationary lens group and accordingly the exit pupil is apt to be positioned close to the image surface. Especially when the lens system is arranged to be compact in size, it becomes more difficult to position the exit pupil remote from the image surface.
When the movable lens group is positioned in front of the stationary lens group to set the focal length shorter (wide angle), the stop is remote from the movable lens group because it is inside the stationary lens group, which is very disadvantageous to correction of aberrations, and accordingly arrangement of the lenses in the movable lens group becomes complicated, whereby the overall lens system becomes larger in length and in diameter of the front lens.